ABSTRACT Healthy lung function is maintained by epithelial cells that comprise the barrier between environmental constituents and host tissues. These cells become dysregulated in chronic inflammatory and fibrotic lung disease, reflecting both genetic and environmental contributions. We have identified a novel association between an abundant environmental polysaccharide and the progression of lung fibrosis. This polysaccharide, chitin, accumulates abnormally in the airways of humans with idiopathic pulmonary fibrosis (IPF), an irreversible, fatal interstitial lung disease that is not completely understood. IPF has been linked to alveolar epithelial cell dysfunction and unknown environmental factors. Chitin is normally degraded by acidic mammalian chitinase (AMCase), a chitinolytic enzyme conserved in mice and humans that is secreted by lung epithelial cells to mediate chitin clearance from the airways. Our results indicate that AMCase is produced by epithelial cells that may be directly involved in the pathogenesis of lung fibrosis. Environmentally-derived chitin spontaneously accumulates in the airways of mice lacking AMCase or bearing IPF-associated genetic mutations, coincident with the development of age-related lung fibrosis in both strains. Additionally, whereas wild-type mice resolve fibrosis after acute epithelial injury, AMCase-deficient mice progress to severe interstitial lung fibrosis associated with increased mortality, resembling key aspects of human fibrotic lung disease that have been difficult to recapitulate in animal models. Together, these data suggest that the routine process of degrading insoluble chitin particles in the airways is dysregulated in lung fibrosis and may be a relevant environmental driver of disease. We have generated several genetic knockout, transgenic, and reporter mice that enable the tracking and functional assessment of relevant lung epithelial cell populations. These different mouse strains will be used to dissect the contributions of the mammalian chitinolytic system and associated epithelial cells to the development of lung fibrosis. We will characterize the molecular mechanisms of chitin degradation using novel assays incorporating common human and mouse enzyme variants on complex substrates. These assays are coupled with in vivo approaches that will allow for rapid testing of therapeutic candidates that can target natural substrates in a lung fibrosis setting. Thus, in this project, we propose coupling therapeutic chitinase development with the study of how chitin drives fibrotic lung disease in three aims: 1. To determine the contribution of environmental chitin to lung fibrosis. 2. To define the cellular and molecular basis for chitinase activity in fibrotic disease. 3. To characterize the mechanisms of chitin degradation mediated by human chitinases and test novel therapeutic approaches in vivo. Understanding ways to efficiently target and degrade chitin in the context of fibrotic lung disease may advance new therapies for a group of related diseases that currently lacks effective treatments.